a. Field of the Invention
The present invention relates to long-life internal-reforming (IR) fuel cells and also to power stations using the same.
b. Description of the Related Art
Reforming fuel cells which use methane gas or the like as a fuel can be divided into two types, one being the internal-reforming type with a reforming means provided inside a fuel cell and the other the external-reforming type with a reforming means disposed outside a fuel cell.
The external reforming type is required to use more fuel gas and also to scale up its reforming means as more cells are used to form a fuel cell having greater generation capacity. Reactions which take place at electrodes of a fuel cell are exothermic reactions, so that the internal temperature of the cell hence arises. This leads to evaporation of an electrolyte or the like, whereby the life of the cell is shortened. To avoid this problem, a cooling means is indispensable. The external reforming type requires a high initial cost as it needs large reforming and cooling means as described above.
On the other hand, the internal reforming type does not require any particularly large reforming means because methane gas is reformed unit cell by unit cell. When unit cells are stacked one over another to permit absorption of heat, which is generated by reactions at their electrodes, by making use of the fact that the reforming reaction is an endothermic reaction, the internal temperature of the cell does not increase. In this manner, the cooling means can also be obviated.
As conventional IR fuel cells, fuel cells of the construction where, with a view toward preventing internal temperature increase of the cells, a reforming catalyst is provided right underneath each anode to permit immediate absorption of reaction heat to be generated by a power generating reaction are disclosed, for example, in Fuel Cell Technology and Applications (International Seminar, The Netherlands, Oct. 26-29, 1987), Extended Abstracts, pages 41 and 45.
The portion right underneath each anode is however a gas flow channel through which both fresh fuel gas and fuel gas after the electrode reaction (i.e., exhaust fuel gas) flow, so that the reforming catalyst is exposed not only to the fresh fuel gas but also to the exhaust fuel gas.
Exhaust fuel gas contains an electrolyte composition and reaction products formed in the electrolyte. Such materials contained in exhaust fuel gas are therefore brought into contact with the reforming catalyst, whereby the reforming catalyst is contaminated. Molten electrolyte and the like, which have flowed down along the surface of an associated separator, also penetrate to the reforming catalyst located below the separator and contaminate the reforming catalyst.
As operation time lengthens, contamination of the reforming catalyst becomes more severe so that the activity of the catalyst is reduced and the conversion from methane gas to hydrogen gas is lowered. As a result, the voltage of the fuel cell drops abruptly.
FIG. 8 diagrammatically illustrates the results of experiments conducted using the same fuel cell, one on generation of power by an internal reforming method in which methane gas was employed as a fuel and the other on generation of power by an external reforming method in which hydrogen gas was fed from the beginning.
Cell voltage is plotted along the axis of ordinates, while operation time is plotted along the axis of abscissas.
As is clearly envisaged from FIG. 8, the open circuit voltage remained substantially constant within a range of 1.0-1.1 V in both the generation of power by the internal reforming method and that by the external reforming method. When a current of 150 mA/cm.sup.2 current density was applied through a closed circuit, the cell voltage did not drop to or beyond 0.7 in the case of generation of power by the external reforming method even when operation time lengthened. In the case of generation of power by the internal reforming method, the cell voltage however abruptly dropped as operation time lengthened and, upon an elapsed time of 330 hours, the cell voltage dropped to 0.4 V and the operation stopped.
As has been described above, the conventional IR fuel cells can theoretically be expected to have longer cell life if temperature increase inside the cells can be prevented. Prevention of temperature increase inside the cells cannot however bring about sufficient effects in practice because performance deterioration of a reforming catalyst proceeds very fast. As a consequence, the IR fuel cells are still accompanied by the drawback that they cannot achieve satisfactory cell life.
Further, a power station making use of such IR fuel cells requires a cell replacement whenever the cell voltage drops, leading to the problem that the efficiency of its operation is low.